This invention relates generally to power generation systems and, more particularly, to a system and assemblies for controlling a temperature of exhaust gases in a stack of a combined cycle power plant.
At least some known combined cycle power generation systems include a gas turbine and a steam turbine in combination to produce power. The power plant is arranged such that the gas turbine is thermally connected to the steam turbine through a heat recovery steam generator (“HRSG”). The HRSG is a non-contact heat exchanger that allows feedwater for the steam generation process to be heated by otherwise wasted gas turbine exhaust gases. The HRSG is a large duct with tube bundles interposed therein such that water is heated to steam as the exhaust gases pass through the duct. The primary efficiency of the combined cycle arrangement is the utilization of the otherwise wasted gas turbine exhaust gases.
Modern combined cycles typically employ two or three steam generation pressures to recover the maximum energy from the gas turbine exhaust. The HRSG heat exchange process is a counterflow process in that the temperature of the hot exhaust gases decreases as they move through the HRSG to the stack whereas the temperature of the steam water mixture in the tubes increases as it is channeled towards the HRSG gas inlet against the flow of hot exhaust gases.
One key parameter in optimizing the combined cycle efficiency is that the highest efficiency is achieved with the lowest stack gas temperature at the outlet end of the exhaust gas stack. The lower limit on stack gas temperature is usually proscribed by the sulfur content in the gas turbine fuel. This is because sulfur compounds condense on the tube bundles at certain relatively low temperatures causing severe corrosion on the tube bundles. It is also known that the dew point of the corrosive sulfur compounds increases with increased concentration of sulfur in the fuel.
At least some known methods for optimizing a combined cycle plant efficiency includes a design of the HRSG and steam system to operate with a stack gas temperature that would prevent low temperature heat transfer surface corrosion commensurate with the highest level of sulfur content in the fuel expected to be burned in the specific application. If fuel is burned with lower fuel sulfur content, the HRSG stack gas temperature cannot be lowered to improve efficiency although the sulfur compound concentration would allow it. Conversely, if the HRSG were designed with stack gas temperature commensurate with the lowest fuel sulfur content to be expected, the plant efficiency would be improved; however, the HRSG heat transfer surface would experience corrosion if the fuel with higher sulfur content were burned. However, such a method narrowly limits the sulfur content of the fuel and consequently narrows the type of fuel capable of being burned.